Polyolefin polymers and polymer blends are known for their versatility and applicability in a wide variety of uses. In particular, many polyolefin polymers, including copolymers of propylene with other α-olefins such as ethylene, are well suited for use in applications requiring good stretchability, elasticity, and strength. Such polymers often comprise a blend of two or more propylene copolymers, and may be manufactured by mechanically blending two or more copolymers, or by in-line reactor blending of the copolymers.
Many polyolefin blends known in the prior art are formed into pellets for intermediate storage purposes before being shaped into articles such as fibers, films, nonwovens, extruded coatings, and molded articles. Some of these compositions, however, are known to exhibit poor pellet stability over extended periods of time, leading to agglomeration of pellets and resulting in poor pourability and flowability of the pellets. While the formation of in-line reactor blends of such polyolefin copolymers has been shown to improve stability properties of the polymer pellets, such pellets still have a tendency to agglomerate during shipping and long-term storage, thus presenting processing issues where free-flowing pellets are required. As a result, many known polyolefin blend pellets are dusted, such as with a low density polyethylene dust, to prevent agglomeration. The use of such dusts may be problematic for certain applications, however, because they may increase gel levels in films, be unacceptable for use in food contact applications, or create housekeeping issues because of increased dust in the plant environment. Further, the typically low melting points of such known polymer blends often lead to flattening or other deformation of polymer pellets during long-term storage, which also negatively affects the ability of the polymer pellets to be free-flowing.
Thus, there is still a need for polyolefin polymer blends having increased melting points while maintaining desirable stretchability, elasticity, and strength properties. Additionally, there is a need for pellets formed from such improved polymer blends which have long-term pellet stability and are free-flowing without being dusted.
It is therefore an object of the embodiments described herein to provide processes for forming polyolefin polymer blends and catalyst systems for use in those processes, where the resulting polymers have acceptable mechanical properties and long-term pellet stability. Ideally, such catalyst system should also exhibit high catalytic activity under (propylene) polymerization conditions. It is further an object of the embodiments described herein to provide polymer compositions made from the processes and catalyst systems described herein. These polymer compositions display advantageously narrow composition distributions and high melting points in comparison to previously known polymers having the same comonomer content. As a result, the polymers described herein exhibit improved properties such as pellet stability, elasticity, and other mechanical properties desired for a variety of end use applications.